Fiber-optic components, including switches, connectors, and mechanical splices, which are used in naval weapons systems typically undergo rigorous mechanical shock and vibration. This shock and vibration significantly affects the performance of the fiber-optic components and, in turn, the naval weapons systems. A capability of detecting any optic changes which occur during mechanical shock and vibration would therefore enhance the overall performance of the weapons system.
Prior art devices for testing fiber-optic components include systems comprising a light source, an optic coupler, an analog detector circuit, and an analog tape recorder. These systems commonly use a commercially available detector circuit which has a power supply. The detector circuit measures the amount of light emitted by converting the light into an electric signal and then amplifying the signal to a voltage which can drive the analog tape recorder.
The devices used to convert the emitted light for these detector circuits comprise photoelectric transducers, which are almost exclusively semiconductor photodiodes. The currents supplied by these receivers are proportional over more than eight orders of magnitude to the light incident upon them when operated in a conducting state, i.e., when the voltage on these devices is kept very small. Thus, the devices which perform the amplification of electrical signals for the electrical circuit should require a low voltage.
One device capable of amplification at a low voltage is an operational amplifier. Operational amplifiers have a feedback branch, and the feedback resistance in this branch must be as high as possible to optimize performance because low voltages become submerged in noise. Typically, the feedback branch has an element with a nonlinear, e.g. exponential, characteristic. This element results in the voltage at the output of the amplifier becoming a logarithmic measure of the light incident upon the photoelectric transducer. The disadvantage of this design is that the upper angular frequency of the transfer range varies with the level of steady light. For the case of low illumination intensities, the signal frequencies of interest become uncontrollably cut. As a result, the detector circuit as a whole has a narrow dynamic range of sensitivity.
Examples of prior art for detector circuits with operational amplifiers include U.S. Pat. No. 4,218,613 by Bletz. Bletz discloses a circuit for amplifying electric signals obtained by a photoelectric transducer as a function of the intensity of illumination upon the transducer. The circuit comprises an operational amplifier connected in series with the photoelectric transducer and having a feedback branch. The feedback branch has an electrical element with a nonlinear characteristic connected in parallel with a resistance. The nonlinear electrical element is either a diode or a transistor having its base at zero potential. This circuit attempts to have optimum noise control with a wide dynamic range.
Another example of prior art is U.S. Pat. No. 4,795,905 by Zierhut, which discloses a circuit layout for an infrared room surveillance detector. This circuit comprises a high impedance operational amplifier connected directly to the terminals of a pyrometer used as an infrared sensor. The reaction resistor of the operational amplifier has a high impedance, preferable in a range of 10.sup.11 to 10.sup.12 ohms. A blocking diode or a transistor connected as a blocking diode connects between the negative terminal of the operating voltage source and the reference voltage output. The circuit has a low noise component in the detector output signal and a constant amplification over a relatively broad frequency range, but is not particularly well-suited for systems testing mechanical shock and vibration because the baseline noise continues to expand throughout the operation.